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| |  Younger People Most Likely to Contract H1N1, But Mortality Rate Highest Among Elderly NEW YORK -- November 11, 2009 -- A study of Mexican influenza A(H1N1) cases shows that while infants and people aged 39 years and under are most likely to get infected, elderly people have the highest H1N1 mortality rates. The data is published online first and will appear in an upcoming edition of The Lancet. Victor Borja-Aburto, Mexican Institute for Social Security, Mexico City, Mexico, and colleagues analysed information gathered by the influenza surveillance system from April 28 to July 31, 2009, for patients with influenza-like illness who attended clinics that were part of the Mexican Institute for Social Security network. They found that, by July 31, 63,479 cases of influenza-like illness were reported. Of these, 6,945 (11%) of cases were confirmed to be H1N1. Of the confirmed H1N1 cases, 6,407 (92%) were outpatients, 475 (7%) were admitted and survived, and 63 (<1%) died. Those aged 10 to 39 years were most affected (56%). Mortality rates showed a J-shaped curve, with greatest risk in those aged 70 years and older (10.3%). The following mortality rates were recorded in other age groups: 60 to 69 years, 5.7%; 50 to 59 years, 4.5%; 40 to 49 years, 2.7%; 30 to 39 years, 2.0%; under 1 year, 1.6%; 20 to 29 years, 0.9%; 1 to 9 years, 0.3%; 10 to 19 years, 0.2%. Risk of infection was lowered by 35% in those who had been vaccinated for seasonal influenza. Every day of delay in hospital admission after the fourth day of symptoms onset increased the risk of death by almost 20%, and presence of chronic diseases in a person increased their risk of death 6-fold. The authors pointed out that pregnant women accounted for 6% of the deaths in their study (4 of 63), slightly lower than the rate recorded in the United States during a similar period (8% or 7 of 87 deaths). “In Mexico, all pregnant workers were sent home during the peak of the pandemic, which probably accounts for this difference,” the authors wrote. “Information about pregnancy in ambulatory confirmed cases was not available in the surveillance system, and we were unable to estimate the case fatality rate for pregnant women. None of the pregnant patients who died received antiviral drugs during the first 48 hours and none had received influenza vaccine. Treatment should begin immediately after onset of symptoms in this group, and vaccination during pregnancy is not contraindicated and therefore can be considered.” Possible protection against H1N1 flu through the seasonal influenza vaccine is, the authors acknowledge, controversial. “The Mexican population who has received seasonal influenza vaccination since 1977, including H1N1 components, could have benefited from cross immunity,” they wrote. “The high incidence of infection in young people could show not only their different exposure related to their daily activities but also that people aged older than 60 years might have some immunity against the H1N1 virus.”
SOURCE: The Lancet
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